A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium for use in a hard disk drive.
B. Description of the Related Art
In a perpendicular magnetic recording medium, mainly a Co—Pt-based alloy is used as a material of a magnetic recording layer for recording data. The Co—Pt-based alloy has a hexagonal close-packed (hcp) structure and shows uniaxial anisotropy in a c-axis direction thereof. The hcp c-axis of the Co—Pt-based alloy needs to be oriented in a direction perpendicular to a principal plane of the magnetic recording medium (disk surface) in order to perform perpendicular magnetic recording. Furthermore, crystal axis orientation dispersion of the Co—Pt-based alloy forming the magnetic recording layer needs to be reduced as much as possible in order to improve the signal-to-noise ratio (SNR) of the magnetic recording medium. In addition, the crystal grain diameter of magnetic crystal grains (magnetic particles) of the Co—Pt-based alloy and dispersion in the grain diameter of the magnetic crystal grains needs to be reduced in order to reduce noise and obtain a favorable SNR. These requirements apply not only to the Co—Pt-based alloy mentioned above, but also to other materials of magnetic recording layers. In the magnetic recording layers made of other magnetic recording materials as well, not only is it necessary to reduce crystal axis orientation dispersion of each magnetic recording layer as much as possible, but also the crystal grain diameter of magnetic crystal grains (magnetic particles) and dispersion of the magnetic crystal grains need to be reduced.
Generally, a plurality of underlying layers are formed under a magnetic recording layer in order to control the orientation and grain diameter of crystal grains of the magnetic recording layer. The underlying layers include (1) an amorphous underlayer having an amorphous structure or microcrystalline structure, (2) an orientation control layer made of NiCr alloy, NiMo alloy, NiW alloy or the like having a face-centered cubic lattice (fcc) structure and for controlling a crystal plane to be grown, and (3) an intermediate layer made of Ru or Ru alloy and assisting in reduction of the number of crystal defects and formation of a high-quality magnetic recording layer. Moreover, as a way to reduce the film thickness of the intermediate layer and the usage of the expensive Ru or Ru alloy, a layer made of Cr or CoCr alloy is often formed between the orientation control layer and the intermediate layer.
For instance, for the purpose of not only reducing crystal axis orientation dispersion of a magnetic recording layer, but also microfabricating the crystal grains and reducing the film thickness of a non-magnetic intermediate layer, the use of the following stacking structure has been proposed as the underslab for forming the magnetic recording layer, the stacking structure being configured by: a first soft magnetic underlayer of an fcc structure that contains Ni and Fe; a first non-magnetic intermediate layer made of Ru and the like; a second soft magnetic underlayer of an fcc structure that contains Co; and a second non-magnetic intermediate layer made of Ru or Ru alloy (see Japanese Patent Application Publication No. 2008-117506). In this proposal, in addition to Co, the second soft magnetic underlayer can contain one or a plurality of types of elements selected from the group consisting of Fe, B, Al, Zr, Mg and Si.
Alternatively, for the purpose of improving the crystal orientation of a magnetic recording layer and microfabricating the crystal grains of the same, the use of the following stacking structure has been proposed as the underslab for forming the magnetic recording layer, the stacking structure being configured by: a seed layer of an hcp structure oriented to (002); a first intermediate layer of a body-centered cubic lattice (bcc) structure oriented to (110); and a second intermediate layer of an hcp structure orientated to (002)(see Japanese Patent Application Publication No. 2009-032356). In this proposal, the first intermediate layer preferably contains Cr of 60 atomic % or more. The first intermediate layer can further contain such elements as Ni and Co.
In addition, for the purpose of reducing the film thickness of an intermediate layer, the use of the following stacking structure has been proposed as the underslab for forming a magnetic recording layer, the stacking structure being configured by: a soft magnetic underlayer of an fcc structure that contains Co, Ni, and Fe; and a non-magnetic intermediate layer containing at least one element selected from the group consisting of Ru, Re, Pd, Ir, Pt, and Rh (see Japanese Patent Application Publication No. 2006-120231).
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.